Three-axis inertial reference sensor



Dec. 16, 1969 N. s. BERS 3,483,746

THREE-AXIS INERTIAL REFERENCE SENSOR Filed Oct. 17, 1966 2 Sheets-Sheet1 (TO/MPVTEE Ala 0701. #4044 J. 556:,

Dec. 16, 1969 N. s. BERS THREE-AXIS INERTIAL REFERENCE SENSOR I 2Sheets-Sheet 2 Filed Oct. 17, 1966 3,483,746 THREE-AXIS INERTIALREFERENCE SENSOR Naum S. Bers, Los Angeles, Calif, assignor to HughesAircraft Company, Culver City, Calif., a corporation of Delaware FiledOct. 17, 1966, Ser. No. 587,050 Int. Cl. G01c 21/00, 19/00, 17/00 US.Cl. 73-178 Claims ABSTRACT OF THE DISCLOSURE The three-axis inertialreference sensor comprises a platform which is gimbaled about twoorthogonal axes of the platform, and which with respect to inertialspace rotates only with rotation of the vehicle about an axis normal tothe plane of the first two axes; that is, this plane of the platform ismaintained in its starting point attitude in space. Angular movementabout the normal axis is measured by a rate gyroscope on the platform.The platform includes a two degree-of-freedom gyroscope directlyconnected to a computer which is spatially removed from the platform.The computer processes the angular and linear information from theplatform sensors to obtain data of the vehicles attitude and translationand generates torques which are fed back into the platform gimbaltorquers to compensate for angular velocity disturbances on the system.

The present invention relates to a three-axis inertial reference sensorincluding a two-axis platform system and, more particularly, to such asensor wherein the platform is free to rotate about an axis normalthereto, relative to inertial space, with the vehicle on which it ismounted, and wherein angular movement about the normal axis is sensedthrough a rate gyroscope.

The Well-known three-axis platform system includes a stabilized elementor platform which is non-rotatably held in position in space by theincorporation of an assembly of three single-degree-of-freedomrate-integrating gyroscopes, or a single two-degree-of-freedom gyroscopeplus one single-degree-of-freedom rate-integrating gyroscope, or twotwo'degree-of-freedom gyroscopes. Three rectilinear accelerometers areincluded within the platform system in order to measure the accelerationof the vehicle in which the platform system is incorporated. Thenon-rotating stable element or inner gimbal is gimbaled along three axesso that any movement of the vehicle to which it is attached will not beimparted to the stabilized element. To ensure the non-rotationalattitude of the inner gimbal, all three gimbals are torqued throughoutput signals from the platform gyroscopes and these signals, whichcontrol the torquing of the outer two gimbals, must be resolved becausethe outer two gimbals rotate with the vehicle. A resolver, therefore, isconventionally used to accomplish this purpose.

Because the above-described stabilized element requires three gimbalsand other apparatus associated therewith to maintain the fixed spatialposition of the stabilized element, several problems arise. Each gimbalmust be journaled in substantially frictionless bearings and theassembly of gimbals must be accurately constructed to insure a properlybalanced, aligned and temperature compensated system; however, anyinaccuracies in any one gimbal may accumulate to impair the properfunctioning of the system. In addition, the resolution of the gyrosignals to control the torquing of the outer two gimbals necessitatesthe use of a relatively complicated resolver. The requirement of suchprecise and complicated equipment increases the possibilities ofmalfunction and, regardless of the degree of precision of the comnitedStates Patent 0 3,483,746 Patented Dec. 16, 1969 ponents, the device maystill retain some of the abovementioned deficiencies. In addition, anyof the several component parts may deteriorate over a period of time toimpair proper performance of the assembled system.

The present invention simplifies and reduces the number of componentparts in the system and, consequently, increases the efficiency thereof.The invention includes a two-axis platform, the plane of said two axesbeing mechanized in such a manner as to prevent rotation of the platformrelative to inertial space about any axis contained within the plane. Itfollows, therefore, that not only is the plane stabilized but also alllines or axes normal thereto are inherently stabilized. By sensing anyrotation about such normal axes or lines, the attitude of the platformmay be determined.

The invention accomplishes this result by gimballing the platform abouttwo axes and by securing an instrument thereto which measures the rateof rotation of the platform about an axis normal thereto. By mechanizingthe three-axis inertial reference sensor in this manner, the resolutionof the gimbal angles is greatly simplified over the well-knownthree-axis platform system such that only a simple cosine angleresolution is required. This resolution and other computations alongwith their associated hardware is spatially removed, that is, placedsubstantially outside of the platform system and its moving parts tosimplify its mechanization.

All the computations are performed in an associated computer whichprocesses the various angular and linear information from the platformsystem so that various data of the vehicles attitude and translation maybe obtained. In addition, the computer generates torques which are fedback into the platform system to compensate for angular velocitydisturbances on the system.

It is, therefore, an object of the present invention to provide asimple, yet reliable, inertial guidance system.

Another object is the provision of an inertial reference sensor ofsimplified construction.

A further object is to provide a sensor of loW weight.

Another object is the provision of an efficient and accurate inertialreference sensor.

Still another object is to provide a system having simple sine or cosinetrigonometric signal resolution means.

Another object is the provision of a novel means for compensating forangular velocity disturbances on the platform.

Other aims and objects as well as a more complete understanding of thepresent invention will appear from the following explanation of anexemplary embodiment and the accompanying drawings thereof, in which:

FIG. 1 schematically illustrates a first embodiment of the inventionincluding a single two-degree-of-freedom gyroscope and associatedelectronic equipment;

FIG. 2 illustrates the rate gyroscope employed in FIG. 1; and

FIG. 3 is a functional block diagram of the embodiment.

With reference to FIG. 1, a three-axis inertial reference sensor 10comprises a two-axis stabilized element or platform 12 which isjournaled within a vehicle 14 about two orthogonal axes YY and ZZ,respectively, one of which is fixed in the vehicle, and which supports atwo degree-of-freedom gyroscope 16 (FIG. 1). A rate gyroscope 22 issecured to platform 12 to measure the rate of rotation thereof about anaxis XX which is orthogonal to the plane of platform 12 defined by axesY-Y and ZZ. Translational information along the axes XX, YY and ZZ isobtained from accelerometers 24, 26 and 28 which are orthogonallypositioned With respect to each other on the platform. Consequently, bycombining the angular information from rate gyroscope 22 and othercomponents associated with the platform and the translationalinformation from the accelerometers,

- within the platform and, therefore, which are related to gyroscope 16or gyroscopes 18 and 20. Furthermore, since gyroscope 16 comprises agyro inner gimbal 30 and a gyro outer gimbal 32, information related togimbals 30 and 32 is respectively designated by the subscript numerals land 2. Similarly, the term platform is used to define the assembly ofall components which have no relative motion with respect to thestabilized plane. In addition, because the platform itself is providedwith two degrees of freedom along axes YY and ZZ by use of an outermostgimbal 34, information arising from relative rotation between platform12 and outermost gimbal 34 is designated by the sumscript I and angularinformation between vehicle 14 and the outermost gimbal is designated bythe subscript O.

This information may be summarized as follows:

A. Input disturbance signals (1) Gyro gimbal torques:

T about axis 31 of inner gimbal 30 T about axis 33 of outer gimbal 32Platform gimbal torques:

T about axis 35 of platform 12 T about axis 37 of outermost gimbal 34Vehicle angular velocities:

o X-axis angular velocity My, Y-axis angular velocity w Z-axis angularvelocity B. Output performance signals (3) Relative platform andoutermost gimbal attitudes:

@ (ii between platform 12 and outermost gimbal 34 (D between outermostgimbal 34 and vehicle 14 (4) Attitude of platform and outermostgimbalPosition, velocity, acceleration:

6 6 6' platform 12 about its axis 35 .0 9 8' outermost gimbal 34 aboutits axis 37 KG(s) servo torques (a frequency dependent multiple of thegyro output signal) wherein K is the gain or amplification factor andG(s) is the signal shaping or filtering characteristic: I I T about axis35 of platform 12 T about axis 37 of outermost gimbal 34 (6) Computer 40generated servo torques:

T about axis 31 of gyro inner gimbal T02, about axis 33 of gyro outergimbal 32 Tcr, about axis of platform 12 T about axis 37 of outermostgimbal 34 C. System parameters 4 J =gyro inner gimbal 3t} moment ofinertia about its axis of rotation I =gyro outer gimbal 32 moment ofinertia about its axis of rotation D =gyro damping coefficient D yrodamping coeflicient (2) Platform 12 parameters:

.l platform 12 moment of inertia about its Z axis I =outermost gimbal 34plus platform 12 moment of inertia about axis of rotation of outermostgimbal 34 K platform 12 spring coefficient K outermost gimbal 34 springcoefficient D platform 12 damping coefficient D =outermost gimbal 34damping coefiicent K =platform inner stabilization loop gain K =platformouter stabilization loop gain (3) Parameters, in general:

s=Laplace operator G(s) :Laplace transform of compensation network:

The various gimbals and the platform are journareu in substantiallyfrictionless bearings with respect to each other and the vehicle in thewell-known manner and, consequently, various journaled connection aredepicted schematically with various signal generators, S and varioustorquers, T, being secured between related bearings and shafts. Thus,signal generator 5 senses the relative angle about a shaft 31 betweengyro inner gimbal 30 and gyro outer gimbal 32 while torquer T provides atorque therebetween. In a similar manner, signal generator S and torquerT are placed between the platform and the gyro outer gimbal on the shaft33, signal generator 5 and torque T are displosed between the outermostgimbal and the platform on a shaft 35 and signal generator S and thetorquer T are positioned between the vehicle and the outermost gimbal ona shaft 37.

An electrical connection 36 leads from signal generator S to a pulsetorquing amplifier 39 having a transfer function K G(s) in order toprovide a torque of the magnitude T to torquer T 101 while an electricalconnec tion 38 leads from signal generator 8 to a pulse torquingamplifier 41 having a function K G(s) for a supply of a torque of themagnitude T to torquer T In each of the pulse torquing amplifiers of thetype KG(s), the output torque is a frequency dependent output multipleof the gyroscopic output signal from the signal generator. Amplifiers 39and 41 are of a Well-known construction and perform the function ofamplification [K] having a shaping characteristic [G(s)] in the form ofa filter.

Signals are furthermore fed to a digital computer 40 of well-knownconstruction by leads 42 and 43 from signal generators 8 8 S and S fromrectilinear accelerometers 24, 26 and 28, and from rate gyroscope 22,these signals being all in incremental form. The signal from the rategyroscope, which is a measure of the angular velocity ta of platform 12,is furthermore processed by computer 40 and is fed back through leads 44into torquers T and T and into amplifiers K G(s) and K G(s), the lattertwo amplifiers combining the computer torques T and T respectively withthe torques T and T aroused by signal generators S and for supply totorquers T and T These torques T01, T02, T and T are used to compensatefor the disturbance angular velocities w my and m as well as for anypredetermined drift of the gyroscope and platform during vehicleacceleration.

Computer 40 processes these and the linear acceleration signals in orderto show the angular and translational data of vehicle 14, as depicted inFIGS. 1 and 3 wherein:

A :Accelerati0n in the vehicles transverse direction A =Acceleration inthe vehicles longitudinal direction A =Acceleration in the vehiclesnormal direction V Velocity in the vehicles transverse direction V=Velocity in the vehicles longitudinal direction V =Ve1ocity in thevehicles normal direction P =Position in the vehicles transversedirection P Position in the vehicles longitudinal direction P =Positionin the vehicles normal direction P=Pitch R=Roll Two-degree-of-freedomgyroscope 16, in addition to having orthogonally journaled gimbals 30and 32 is provided with a rotor 46 which is journaled on a shaft 47 in awell-known manner about its spin axis in gyro inner gimbal 30.

Preferably, damping means not shown, but represented in the drawing bynumerals 48 and 50 in the form of a viscous fluid is placed betweengimbals 30 and 32 and between platform 12 and gyro outer gimbal 32,respectively, for purposes of adequate stability.

Rate gyroscope 22 (see FIG. 2) is of conventional designand comprises arotor 52 journaled on a shaft 53 about its spin axis in a gimbal 54which, in turn, is journaled in a gyro case 56. A signal generator 58 issecured between the case and gimbal 54 to sense the relative anglestherebetween and to feed this angular information to computer 40 throughlead 43 and to pulse torquing amplifier 60 to provide a pulse torquingrestraint on gyroscope 22. Amplifier 60 energizes a torquer 62 which isalso connected between case 56 and gimbal 54. Amplifier 60 is preferablypulse torqued so that the Euler angle about axis XX may be generatedthrough a pulse counting integration; however, an electronic integratormay be employed in order that the angular velocity about axis XX, tax,may be taken from gyroscope 22 as shown in FIG. 2. As in the case withgyroscope 16, rate gyroscope 22 is provided With a damping means notshown, but represented in the drawing by numeral 64 which is placedbetween gimbal 54 and case 56.

Referring now to FIG. 3, sensor 10 of FIG. 1 is illustrated infunctional block diagram form wherein the various signals and parametersare defined as above. The sensor, as depicted in the block diagram,includes an inner stabilization loop 70 comprising a sensing portion 72and a control portion 74 and an outer stabilization loop 76 comprising asensing portion 78 and a control portion 80.

Sensing portion 72, which derives its information primarily fromtwo-degree-of-freedom gyroscope 16, includes a pair of summingamplifiers 82 and 84 and a pair of integrators 86 and 88 to provide therelative gyro gimbal position between gyro inner and outer gimbals 30and 32 which position is fed to computer 40 and to control portion 74.Both the summing and integration are performed by use of conventionalgyroscopic apparatus, the integration being an inherent function of thegyroscope as is well-known in the art.

Inner stabilization loop portion 74 through pulse torquing amplifier 39,thereafter provides a torque T; which, as combined with the computergenerated torque T is exerted on platform 12. Th relative angular position between platform 12 and outermost gimbal 34 is produced byportion 74 which position is also fed to the computer. A comparison ofFIGS. 1 and 3 therefore illustrates the manner in which information fromsignal generator S moves to amplifier 39 and torquer T C S ensingportion 78 of outer stabilization loop 76 operates in a manner similarto that of sensing portion 72 to provide the relative gyro gimbalposition between gyro outer gimbal 32 and platform 12. Position is fedto computer 40 and to control portion 80 which, in turn, produces therelative platform gimbal position Position is also fed into thecomputer. The connections of portions 78 and 80 parallel the flow ofinformation from signal generator S into amplifier 41 and torquer T Theblock diagram of FIG. 3 also depicts the inputs of several angularvelocity disturbances such as x @0 Sin r+ z 5111 @0 $511 (91) and (cosThese trigonometric inputs result, in part, from the fact thatinformation relating to axes X and Z are taken from platform 12 andoutermost gimbal 34 both of which are gimbaled with respect to axis Y.

Since rate gyroscope 22 is positioned on platform 12, th angularvelocity w about axis X affects loops 70 and 76 only through computer40. Thus, the electronic apparatus on sensor 10 itself is greatlysimplified because the electronic equipment associated with the rategyroscope may be incorporated in computer 40 and, therefore, theefiiciency of the sensor is increased over and made more reliable thanth well-known three-axis platform system.

Although the invention has been described with reference to a particularembodiment thereof, it should be realized that various changes andmodifications may be made therein without departing from the spirit andscope of the invention.

What is claimed is:

1. A three-axis inertial reference sensor comprising computational meansand a two-axis stabilized element having gimbal means, torquers andsignal generators on each of the two axes thereof and rotatable withrespect to inertial space about an axis normal to the plane of the axesof stabilization of said element; a two degree-offreedom gyroscope and arate gyroscop for measuring the rate of rotation of said element aboutthe normal axis, both gyroscopes being supported by said element; saidtwo degree-of-freedom gyroscope having gimbal means, torquers and signalgenerators; said torquers and signal generators being connected to saidgimbal means, directly connected to said computational means and respectively transmitting and receiving signals to and from saidcomputational means.

2. A three-axis inertial reference sensor as in claim 1 wherein saidrate gyroscope is rigidly secured to said element.

3. A three-axis inertial reference sensor as in claim 1 wherein saidrate gyroscope comprises a rate integrating gyroscope provided with apulse-torquing restraint.

4. A three-axis inertial reference sensor as in claim 1 furtherincluding three rectilinear motion accelerometers connected to saidelement for sensing translational motion thereof.

5. A three-axis inertial reference sensor as in claim 1 wherein saidcomputational means includes a computer spatially removed from saidelement.

References Cited UNITED STATES PATENTS 2,914,763 11/ 1959 Greenwood.2,949,785 8/1960 Singleton et al. 3,127,774 4/ 1964 Fischer et al.3,229,533 1/1966 Draper et al. 3,258,977 7/1966 Hoffman. 3,281,58110/1966 Lerman et al. 3,284,617 11/1966 Lerman. 3,310,876 3/1967 Yamron.3,365,942 1/ 1968 Blazek et al.

FOREIGN PATENTS 379,135 8/1964 Switzerland.

ROBERT B. HULL, Primary Examiner U.S. Cl. X.R.

